The invention relates to an array antenna for use in a base station of a mobile communication such as automobile telephone, cellular telephone or the like and comprising an array of a plurality of antenna elements to provide a service area defined by an angular range in a horizontal plane or a so-called sector area, and more particularly, to an adaptive array antenna unit having an adaptive processor which adaptively suppresses an interference wave connected thereto.
In the mobile communication such as automobile/cellular telephone or the like according to the cellular system, those base stations which are distantly spaced apart utilize identical frequencies in order to increase the subscriber capacity so that limited frequencies can be efficiently utilized. However, when frequencies are used repeatedly, there arises a problem of interference noises due identical frequencies. Another issue occurs that the subscriber capacity is degraded as the interference noises increase.
Conventional approach to suppress the interference noises has been the use of a directional antenna for the base station antenna. An antenna which exhibits the directivity in the horizontal plane is utilized, and techniques such as sectoring a cell or a beam tilting which varies the directivity in the vertical plane have heretofore been employed. These techniques achieve the effect of improving the reception SIR (signal wave/interference wave ratio) in that the use of a directional antenna for the base station antenna is effective to suppress interference waves from directions other than the direction of the antenna directivity.
In addition to these techniques, an investigation is recently being made to suppress interference noises by the use of an adaptive array antenna. An adaptive array antenna refers to the technique which employs a plurality of antennas (an array antenna) arranged so as to be spatially spaced apart to define adaptively a directivity having null beam (of zero sensitivity) in the direction of an interference wave and a narrow beam in the direction of a desired wave, thus suppressing the interference noise level. However, in the investigation of past adaptive array antennas, it is desired that the beam direction thus defined can be changed at will over a broad range, and accordingly, a non-directional (or whole directivity: omni-directivity) element is used for each of the antenna elements. An arrangement in which a directional antenna is used for individual elements which constitute together an array antenna to provide their radiant directivity is scarcely found. Even in the CDMA system, there has been no idea of employing an adaptive array antenna which uses directional antenna elements.
As mentioned previously, a sectoring technique is frequently employed in the cellular system, and a directional antenna which is adapted to the sectored configuration is required at this end. In a conventional system which does not employ an adaptive array antenna, an antenna of a base station has a directivity in a horizontal plane, a half power width (hereafter referred to as beam width) of which is equal to a sector width. Thus, an antenna having a beam width of 120xc2x0 is normally used for a 120xc2x0-sector (or 3 sector) arrangement. In an investigation which deals with the application of a directional antenna to a prior art base station adaptive array antenna (see xe2x80x9cInfluences of antenna directivity in a mobile communication base station adaptive array antennaxe2x80x9d by Ryo Yamaguchi and Yoshio Ebine, Academy of Communication Technical Report AP 96-131, 1997-01), it is reported that an antenna having a beam width broader than the sector angle is required to construct sectors since the angle over which interference waves can be rejected is narrower than the beam width of the antenna. The investigation disclosed in this literature relates to a mobile communication system which incorporates TDMA system as the radio access technique, and thus reveals an outcome of investigation obtained under a condition that there are a relatively few number of interference waves. Currently, there is no instance of investigating a relationship between the sector angle and the beam width under a condition that there are an increased number of interference waves as in the CDMA system.
Thus, the use of a directional antenna has little been taken up in the investigation of conventional adaptive array antennas, and accordingly, there has been little disclosure on how an optimum antenna can be constructed when an adaptive array antenna is to be used with a sector cell for which a directional antenna is used. In particular, it is the current status of the art that no antenna arrangement has been disclosed which can be used in an environment that a number of interference waves are oncoming from all directions as occurs in a system which incorporates the CDMA as the radio access technique.
It is an object of the invention to overcome such problem and to provide an optimum adaptive array antenna unit for a base station according to the CDMA mobile communication system.
According to a first aspect of the invention, in an adaptive array antenna unit for a base station of mobile communication in which CDMA system is employed as the radio access technique, a service area within a sector is defined by using antenna elements which constitute together an array antenna and each have a beam width within the horizontal plane which is narrower than the sector angle. In particular, the service area can be defined by a number of antenna elements greater than the number of antenna elements (referred to as reference number) which is required when the beam width within the horizontal plane of the antenna element is substantially equal to the sector angle.
According to a second aspect, an antenna having a beam width broader than the sector angle within the horizontal plane is employed as an element. In particular, the service area can be defined by a number of antenna elements which is reduced from the reference number of elements.